1. Field of the Invention
This disclosure relates to high-wear nozzles for directing the flow of solid particles entrained in a fluid stream.
2. Discussion of Related Prior Art
A solid-fueled firing system burns powdered solid fuel; typically coal, blown into a furnace in a stream of air. This furnace is typically a boiler that creates steam for various uses, such as creating electricity. FIG. 1 shows a stationary solid fuel nozzle assembly 100 and a coal nozzle tip 130 used to inject the pulverized solid fuel into the boiler. The pulverized solid fuel is provided by coal piping (not shown) that attaches to a round flange 140. The function of the stationary nozzle 100 is to direct a coal air mixture into the nozzle tip 130 and then into a combustion chamber of a furnace.
The coal piping and the round flange 140 have a round cross-sectional shape. The nozzle tip 130 has a rectangular or square cross-sectional shape. Therefore, the cross-sectional geometry of the nozzle 100 must change from round at its inlet 103, to a square flange to mate with the coal nozzle tip 130.
A transition section 110 between the inlet 103 and the nozzle tip 130 must accelerate the coal mixture as it passes through the nozzle 100 from a transport velocity at its round inlet 103 to a predetermined higher velocity required for optimum performance at the coal nozzle tip 130.
Powdered coal itself is not very abrasive; however, the impurities in the coal, such as ash and silica can be very abrasive. The impurities can comprise typically 10-20% of the coal composition. Therefore, blowing the pulverized solid fuel through the nozzle 100 can have the same effect as sandblasting. The high velocity abrasive coal-air mixture therefore causes rapid erosion of the coal nozzle 100 and nozzle tip 130.
Whenever an electricity producing plant is not in operation, the power plant operator is required to buy electricity from alternative sources for its customers. This can become very costly.
The coal nozzle 100 is located within the boiler windbox, so removal, repair and replacement is time-consuming and expensive. Removal requires that the boiler be shut down and cooled. The access panels on the windbox are removed and the coal piping disconnected. All this takes time so coal nozzles 100 can only be replaced during a relatively long shutdown. Due to the expense and difficulty of replacing these, a long service life is extremely important. Previous attempts to maximize service life of the stationary coal nozzle consisted of using thick cast iron structures or lining the stationary coal nozzle with ceramic tiles. There were shortcomings with these prior art methods of reducing wear in the nozzles 100.
Even with the increased thickness cast iron nozzles, they did not last much longer. The abrasion resistance of cast iron is much lower than typical wear protecting materials. The cast iron coal nozzles rarely last more than 3-6 years, depending on the coal type burned. This limited the length of time a boiler could be used without an extended maintenance shutdown. If the cast iron coal nozzle 100 is damaged, repair is also difficult because cast iron is difficult to weld. Also, cast iron nozzles are very heavy and difficult to transport and install.
Ceramic is more wear resistant than cast iron. Therefore, lining the nozzles with ceramic tiles increased the usable life of the nozzles. However, these tiles are difficult to attach to the nozzle inner surface. They are typically plug welded, and tend to detach easily. Due to the odd geometry of a transition from a round cross-sectional shape to a square cross-sectional shape, it is difficult to create tiles that fit tightly together.
It is important to understand that the ceramic tiles are typically arranged in a roman arch design. Once one tiles is lost, the remaining tiles are no longer held in place and also are lost.
Coefficients of expansion are vastly different from ceramic to steel. This means that large temperature fluctuations are likely to cause separation between ceramic tiles and the nozzle surface. This causes the tiles to fall off leaving the nozzle base material unprotected.
An additional shortcoming is that the ceramic tile system is relatively fragile. If lifted by fork trucks or similar lifting equipment there is a possibility that they will flex and crack tiles. They are frequently damaged in transport to the site or during installation. Once damaged, the tile cannot be replaced in the field.
Ceramic tiles can become dislodged and fall off when the coal nozzle buckles due to increase loads. Tiles also become dislodged when customers use large vibrations or percussions to dislodge slag from the equipment within the boiler.
Coal particles sometimes build up in the nozzles. Combustion sometimes propagates down the nozzle creating what are known as furnace “puffs”. These create vibrations that also may dislodge or damage ceramic tiles.
Furthermore, if damage occurs, it may be internal and not visible. This creates the potential that cracked tiles may be unknowingly installed in the nozzles 100. There is a possibility that the nozzles may wear through and cause a windbox fire. This can lead to major damage and repair costs to the facilities and a possibility safety hazard.
Currently, there is a need for an economical, easily constructed, wear-resistant nozzle for directing a stream of abrasive particles entrained in a fluid.